Bug 1357754 - Hook up chaining layer trees. r=botond,jrmuizel

When APZ traverses a tree using LayerMetricsWrapper, the layer tree
already has its RefLayers connected because of the in-scope
AutoResolveRefLayers. When traversing using the
WebRenderScrollDataWrapper though, this is not the case, and we need to
explicitly jump from one layer tree to another during the walk.
Thankfully we don't require upwards traversal in the tree or this would
be much more complicated.

MozReview-Commit-ID: 8gbvUlzghLx

gfx/layers/wr/WebRenderBridgeParent.cpp

@@ -307,10 +307,18 @@ WebRenderBridgeParent::HandleDPEnd(const gfx::IntSize& aSize,
                            dl, dlDesc, aux, auxDesc);
   HoldPendingTransactionId(mWrEpoch, aTransactionId);
 
+  mScrollData = aScrollData;
   // TODO: pass the WebRenderScrollData to APZ (this will happen in a future
   // patch)
 }
 
+const WebRenderScrollData&
+WebRenderBridgeParent::GetScrollData() const
+{
+  MOZ_ASSERT(mozilla::layers::CompositorThreadHolder::IsInCompositorThread());
+  return mScrollData;
+}
+
 mozilla::ipc::IPCResult
 WebRenderBridgeParent::RecvDPEnd(const gfx::IntSize& aSize,
                                  InfallibleTArray<WebRenderParentCommand>&& aCommands,

gfx/layers/wr/WebRenderBridgeParent.h

@@ -160,6 +160,8 @@ public:
     return mIdNameSpace;
   }
 
+  const WebRenderScrollData& GetScrollData() const;
+
 private:
   virtual ~WebRenderBridgeParent();
 
@@ -223,6 +225,9 @@ private:
   bool mPaused;
   bool mDestroyed;
 
+  // Can only be accessed on the compositor thread.
+  WebRenderScrollData mScrollData;
+
   static uint32_t sIdNameSpace;
 };
 

gfx/layers/wr/WebRenderScrollDataWrapper.h

@@ -7,6 +7,8 @@
 #define GFX_WEBRENDERSCROLLDATAWRAPPER_H
 
 #include "FrameMetrics.h"
+#include "mozilla/layers/CompositorBridgeParent.h"
+#include "mozilla/layers/WebRenderBridgeParent.h"
 #include "mozilla/layers/WebRenderScrollData.h"
 
 namespace mozilla {
@@ -157,6 +159,19 @@ public:
       size_t subtreeLastIndex = std::min(mContainingSubtreeLastIndex, prevSiblingIndex);
       return WebRenderScrollDataWrapper(mData, mLayerIndex + 1, subtreeLastIndex);
     }
+
+    // We've run out of descendants. But! If the original layer was a RefLayer,
+    // then it connects to another layer tree and we need to traverse that too.
+    // So return a WebRenderScrollDataWrapper for the root of the child layer
+    // tree.
+    if (mLayer->GetReferentId()) {
+      CompositorBridgeParent::LayerTreeState* lts =
+          CompositorBridgeParent::GetIndirectShadowTree(mLayer->GetReferentId().value());
+      if (lts && lts->mWrBridge) {
+        return WebRenderScrollDataWrapper(&(lts->mWrBridge->GetScrollData()));
+      }
+    }
+
     return WebRenderScrollDataWrapper();
   }